1. Field of the Invention
This invention relates to methods of electroplating or electrodepositing a metal, particularly iron and iron alloys, onto a target substrate where the electroplating solution includes an iron-chelating agent, which forms an iron(III) complex with a higher stability constant than a corresponding iron(II) complex. The iron(III)-chelating agent complex undergoes an electrochemical reduction to produce an iron(II)-chelating agent complex, which then reacts with iron(III) to produce an iron(III)-chelating agent complex and an iron(II) species, which may then be electroplated onto the target substrate. This catalytic cycle works particularly well with siderophores as the iron-chelating agent, specifically the siderophore desferrioxamine B.
2. Discussion of the Related Art
Electroless and electroplating processes are used to form a wide variety of coatings on various types of substrates. Of particular interest, is the formation of “soft” magnetic alloys on micro-magnetic devices such as magnetic recording heads and thin-film inductors and transformers. Using photolithographic photoresist techniques, desired patterns and shapes may be electroplated onto a target substrate.
An electroplating solution may include a source of metal cations for plating, such as an easily dissociated salt of the metal or metals to be plated, for example, the sulfate or the cyanide, and a buffer to maintain the desired solution pH. A complexing agent to lower the concentration of the free metal ion, and thus slow the plating process may also be present in a typical electroplating solution. Retarding the plating process prevents a rough or brittle metal plating from forming on the plated substrate.
Electroplating may be utilized to plate films composed of complex alloys, such as CoFeX, NiFeX, and CoNiFeX alloys, where X includes such elements as P, B, S, or Mo. Typically, the X elements are added to the electroplating solution in the form of oxo-acids or salts thereof. See, for instance, U.S. Pat. No. 5,883,762.
For iron electroplating solutions, formation of ferric or Fe(III)-containing compounds is undesirable since they reduce the plating and current efficiency of the iron electroplating process. The reduction in current efficiency due to the build-up of Fe(III)-containing compounds severely decreases the time that a particular iron electroplating solution may be utilized to electroplate or electrodeposit material onto a target surface. Ferric ions also often precipitate out of the electroplating solution. The desired iron species for plating is ferrous or Fe(II), which is unstable in the presence of, dissolved dioxygen (O2) and may be oxidized to ferric ions at the anode.
Clearly, there is a need for electroplating methods, which decrease the formation of ferric (Fe(III)) species, preferably to ppm levels or lower, and concurrently increase the concentration of ferrous (Fe(II)) species available for plating in an electrodeposition solution.